1. Field of the Invention
This invention relates to the art of optical information and more particularly, to an improvement in optical information record mediums such as digital audio disks, optical video disks, CD video disks, and memory disks for computers such as CD-ROM, CD-I and the like.
2. Description of the Prior Art
Known optical information record mediums have an arrangement which includes a resin substrate which is preferably optically transparent. The substrate is encoded with information signals in the form of pits directly thereon or through a cured lacquer layer at least one side of the substrate. The encoded substrate is subsequently covered on the at least one side with a reflection thin layer of, for example, aluminium. This layer has usually a thickness of 50 to 100 nm. The reflection thin layer is then covered with a protective resin layer having a thickness of approximately 6 micrometers. In practical applications, a print or ink layer is further formed on the protective resin layer.
However, these known information record mediums or disks are disadvantageous in that when they are placed under relatively severe conditions of high temperature and high humidity or are allowed to stand over a long term, the reflection thin film made of Al deteriorates so that the reflectance gradually decreases. This may eventually bring about passage of light or a laser beam, making it difficult to read out the information signal reliably. In a worst case, the medium cannot stand further use.
The above disadvantage may become more pronounced when the medium is colored as required in certain cases. For the coloration of the medium, it is usual to color both the substrate and the protective layer. For this purpose, dyes are added to compositions for the substrate and the protective layer. Such compositions generally comprise, aside from starting resins and dyes, dispersants which allow good dispersion of the dyes in the resin matrix. Such dispersants will adversely affect the reflection thin layer, thereby facilitating the layer to deteriorate. This is considered for the reason that when the Al film contacts the dye or other additives in the substrate and the protective layer, it is more liable to corrode.